1. Field of the Invention
The invention relates to an apparatus and a process for producing a crystal of semiconductor material. The apparatus comprises a crucible and an induction heating coil provided for melting a feed of semiconductor material and for stabilizing a melt of semiconductor material covering a growing crystal of semiconductor material.
2. Description of the Related Art
There is a requirement for large amounts of semiconductor material, particularly silicon, in crystalline form for the production of electronic components or solar cells. The crystals are available in monocrystalline or polycrystalline grades of circular, rectangular or square cross section. The production of crystals having a non-circular cross section is described, inter alia, in US 2005/0188918 A1 and EP 2692908 A1.
Monocrystals of circular cross section are produced on an industrial scale using, in particular, the CZ process and the FZ process. The CZ process comprises pulling a monocrystal growing from a seed crystal, out of a melt contained in a crucible. The melt is generated by filling the crucible with chunks of semiconductor material and melting the chunks using a resistance heating means disposed around the crucible.
The FZ process comprises using an induction heating coil to generate a zone of molten semiconductor material between a seed crystal and a feed rod. The induction heating coil is a flat coil comprising a coil body which is configured to have a hole in the middle, the hole having a specified diameter. Lowering of the seed crystal and feed rod causes the melt zone to move into the feed rod, and semiconductor material melted from the feed rod crystallizes on the seed crystal. A neck section is initially allowed to crystallize to obtain dislocation-free semiconductor material. A monocrystal of semiconductor material subsequently grows to form a cone-shaped section with increasing diameter and ultimately to form a cylindrical section of approximately constant diameter. The FZ process is described in detail in EP 2 679 706 A1 for example.
There are also descriptions of processes similar to the FZ process but differing in that they employ a feed of semiconductor material granules instead of a solid rod. A representative example of such processes is U.S. Pat. No. 5,367,981 which describes the production of monocrystals and polycrystals. It has not yet proved possible to employ any of these processes on an industrial scale. One reason for this is the difficulty of controlling the transport of the melt out of the crucible.